Mac Mail HTML signature headaches

Hallo Mac Community.

I hope that someone can help with a fix for the problems caused by installing an HTML signature in Mac Mail.

We created a signature for a client which works 100% for Gmail and Outlook, but if installed in Mac Mail through a simple copy-paste process from the browser, Mac Mail seems to change the code and we lose the sizing properties of linked images. Ironically enough we made the images higher resolution to display crisply on Mac's retina displays. Mac Mail somehow breaks the sizing instruction in the HTML the minute it is saved and seems to then display it by gross pixel width.

There is a very complicated work-around in which the user needs

1. to create a new signature in Mail

2. go to the new signature .mailsignature file in the hidden library folder Mail

3. paste the html code directly into it under the Mail-created header

4. save the .mailsignature file

5. lock the .mailsignature file so that Mail cannot change the code

Please tell me there is a simpler way to solve this problem. Anyone?